By virtue of excellent magnetic properties, Nd—Fe—B base permanent magnets find an ever increasing range of application. In the field of rotary machines such as motors and power generators, permanent magnet rotary machines using Nd—Fe—B base permanent magnets have recently been developed in response to the demands for weight and profile reduction, performance improvement, and energy saving. The permanent magnets within the rotary machine are exposed to elevated temperature due to the heat generation of windings and iron cores and kept susceptible to demagnetization by a diamagnetic field from the windings. There thus exists a need for a sintered Nd—Fe—B base magnet having heat resistance, a certain level of coercive force serving as an index of demagnetization resistance, and a maximum remanence serving as an index of magnitude of magnetic force.
An increase in the remanence (or residual magnetic flux density) of sintered Nd—Fe—B base magnets can be achieved by increasing the volume factor of Nd2Fe14B compound and improving the crystal orientation. To this end, a number of modifications have been made on the process. For increasing coercive force, there are known different approaches including grain refinement, the use of alloy compositions with greater Nd contents, and the addition of effective elements. The currently most common approach is to use alloy compositions in which Dy or Tb substitutes for part of Nd. Substituting these elements for Nd in the Nd2Fe14B compound increases both the anisotropic magnetic field and the coercive force of the compound. The substitution with Dy or Tb, on the other hand, reduces the saturation magnetic polarization of the compound. Therefore, as long as the above approach is taken to increase coercive force, a loss of remanence is unavoidable.
The method capable of meeting both remanence and coercivity is proposed in Patent Documents 1 and 2. A sintered magnet body of R1—Fe—B base composition wherein R1 is at least one element selected from rare earth elements inclusive of Y and Sc is coated on its surface with a powder containing an oxide, fluoride or oxyfluoride of R2 wherein R2 is at least one element selected from rare earth elements inclusive of Y and Sc. The coated magnet body is heat treated whereby R2 is absorbed in the magnet body.
This method is successful in increasing coercive force while significantly suppressing a decline of remanence. Still some problems must be overcome before the method can be implemented in practice. Means of providing a powder on the surface of a sintered magnet body is by immersing the magnet body in a dispersion of the powder in water or organic solvent, or spraying the dispersion to the magnet body, both followed by drying. The immersion and spraying methods are difficult to control the coating weight (or coverage) of powder. A short coverage fails in sufficient absorption of R2. Inversely, if an extra amount of powder is coated, precious R2 is consumed in vain. Also since such a powder coating largely varies in thickness and is not so high in density, an excessive coating weight is necessary in order to enhance the coercive force to the saturation level. Furthermore, since a powder coating is not so adherent, problems are left including poor working efficiency of the process from the coating step to the heat treatment step and difficult treatment over a large surface area.
As the method of efficiently and tightly depositing a powder of R2 onto the surface of a sintered magnet body, one effective method contemplated is by immersing the magnet body in an electrodepositing solution having the R2 powder dispersed therein, and causing the R2 powder to deposit on the magnet body via electrodeposition. The electrodeposition process enables to control the coating weight of the powder and to form a uniform powder coating having tight adhesion. However, since rare earth elements as typified by Dy and Tb are rare and very expensive, there is still a need for efficient and economical means of coating a rare earth magnet body with a rare earth-containing powder.